FUNGAL MEDIATION OF A PLANT HERBIVORE INTERACTION IN AN EARLY SUCCESSIONAL PLANT COMMUNITY

We used a series of fenced exclosures to examine whether herbivory by mammals affected the structure and dynamics of an old-field plant community in Virginia. Early in the 3rd yr of the study purple clover, Trifolium pratense, a dominant member of the plant community, occurred at higher densities where herbivores were excluded relative to control plots. Following a 1-mo time lag, Trifolium became infected by the rust-forming fungal pathogen Uromyces trifolii; the incidence of infection was higher in exclosures relative to control plots where herbivores were present. Early-to-late season changes in the abundance of Trifolium were related to the degree of fungal infection and not herbivore presence/ absence. Data from the following year (when the fungus and Trifolium were rare) showed clover abundance to be higher in control plots than in exclosures. These results suggest that the negative influence of the fungus not only persisted through time, but more strongly affected host plant abundance in a 2-mo period than did herbivory that occurred in both years. While the direct effect of herbivory on food plants is negative, by keeping plant abundance below that at which fungal infections become epidemic, the net effect of herbivores on some forage plants may be positive. The existence of strong indirect interactions is further evidence that the emphasis in community ecology (and in plant succession) should be on interactive networks rather than on species pairs.